Re: [DML] Lotus Esprit Rear Suspension - very long and boring ;)

[mike.griese@xxxxxxxxxxxxxxxx]

Tom - I don't think your analysis is correct.  The
rear hub is suppposed to travel in an arc perpendicular 
to the frame, just like the front hub.  If it is not, there
is something wrong.  You have to consider the
upper and lower links in your analysis, and they are 
not mentioned in your text.  They define the 
shape of the arc.  The purpose of the trailing
arm is to limit front/rear motion of the hub - not provide
the primary pivots for the suspension.  You also
have to consider how the front trailing arm and upper
shock mounts work.  They are both essentially
point mounts, not axes like the upper and lower
links on the rear or the lower control arm at the
front.  They are designed to allow more degrees
of freedom to account for the bending forces
you are identifying.

What concerns me about people talking about 
fixing the free end of the trailing arm bolt is 
that you change that joint from a point to an 
axis.  When you do that, you get the motion 
you describe at the hub - an arc parallel to 
the frame.  The end of the trailing arm bolt
is free to allow the trailing arm to twist 
slightly as the hub moves up and down.  The
shape of the trailing arm and the way it 
is mounted to the hub is the clue as to 
how the front mount works.  That big, 
wide base pinned to the hub perpendicular
to the arc in which the hub moves plus
the single bolt at the front means that
front joint can't be an axis.  It has
to allow the trailing arm to twist.
The bushing there absorbs the parallel
arc motion, keeping the upper and lower
links (and the shock mount) in the proper
paths of motion.  Otherwise you put all 
of the twisting energy back into the 
upper and lower links, which are on axes.

If I had about an hour with you, my bare Esprit
Esprit frame and a torque wrench, this would
become a lot more obvious.  If you remove the
shock and disconnect the TAB, the hub moves
in an arc perpendicular to the frame.  Next, 
reconnect the TAB and disconnect the upper 
and lower links at the hub and see how the 
trailing arm and hub are allowed to move - it's 
not an arc, it's a cone.

The trailing arm bolts of the Esprit are not
a failure point like they can be on the 
DeLorean.  I think the differences in the 
way the shims are mounted is the main reason.
If that joint is properly torqued (and stays
that way), the shear stresses on the bolt 
are not as big as you might think - they are 
distributed into the bushing.  Once that joint 
loosens (as in a shim falling out), then the 
bushing no longer works and the shear forces 
go into the bolt, causing the bending.

--
Mike

-------------- Original message from "Tom Niemczewski" <dmctom@xxxxxxxxx>: -------------- 


> Mike, 
> 
> Look closely at the lower shock mount on both the front and rear. Both, the 
> lower control arm and the rear trailing arm travel in an arc. The difference 
> here is that the front arm moves perpendicular to the frame while the rear 
> moves in parallel. 
> In both cases the upper shock mount is part of the frame and is fixed. We 
> can omit that small busing that's there in this example. 
> Lets look at the front first. The bolt that supports the shock at the lower 
> mounting point is parallel with the axis on which the control arm rotates. 
> In this design the bolt holding the shock rotates inside the bushing as the 
> suspension travels from top to bottom - the bolt is fixed to the control arm 
> while the shock is fixed to the frame by the upper mount. This joint works 
> as it should. The arc created by the movement of the control arm simply 
> causes the shock mount to rotate. Great! 
> Now, lets look at the rear shock mount. The bolt that supports the shock 
> runs along the line of the vehicle and it is turned 90 degrees to the axis 
> on which the trailing arm rotates. Well, it's not exactly 90 degrees, but 
> close to it. So, as the rear suspension travels from the lowest to the 
> highest point of it's travel the shock bushing cannot rotate on that bolt. 
> Try to imagine what happens with that bolt... with the trailing arm at the 
> lowest point that supporting bolt is actually pointing down. When the 
> control arm moves to the highest point of travel the bolt will point up. 
> This causes bending of the rear shock and very quickly crushes and wears out 
> the lower shock bushing. Now, if this bolt was rotated 90 degrees to be 
> parallel with the TAB the shock would simply rotate on the bolt without 
> damage. 
> Yes, I agree that this setup works. But just because the car doesn't fall 
> apart and can be driven doesn't mean that this is in any way a good design. 
> Actually, in my opinion this design is terrible! What would happen if they 
> would weld the hubs directly to the frame? Would that work... well, yes. 
> Would it be a good design? No way. By saying that is doesn't work I didn't 
> mean that it doesn't work at all... because it does as we all know. It was 
> an exaggeration to make my point. 
> 
> I sure hope that all this makes sense ;) It's sometimes hard to put all that 
> stuff into words. If what I'm saying does not make any sense, please go out 
> to the car, look at the rear shock lower mount and imagine what happens to 
> it as the trailing arm moves up and down drawing an arc at where the shock 
> is mounted. 
> 
> When it comes to the trailing arm mount to the frame and the famous TABs I 
> have to disagree. The trailing arm should not twist on the mount as it moves 
> up and down. Twisting in this joint will cause instability and a feeling of 
> "floating" where it might be difficult to keep the car in a straight line on 
> uneven pavement. Same thing happens when it comes to the fore/aft movement. 
> Besides, the current design puts an awful amount of stress on those TABs. 
> The bolt should be supported from both ends with moving joint in the middle. 
> The engineers at Lotus should now that what they designed is not the way it 
> should be done! It is a critical joint with immense forces at work... To me 
> it seems that whoever designed and approved this joint has never taken any 
> classes in joint design, forces at play and most of all lacked common 
> engineering sense. I know, I'm harsh... but this is what I think when I look 
> at this joint. Especially in such an important joint as this one and with 
> the rear wheels being driven on top of that. Of course the joint cannot be 
> rigid or something will have to give eventually. That is why the bushing is 
> needed on the TAB. I am changing this unfortunate Lotus design to something 
> that will be a lot stronger, safer and lower maintenance. 
> BTW, it there any other car on the road beside the DeLorean that you have to 
> check the TABs? Have you ever seen a TAB from other cars that were severely 
> bent out of shape just with regular use? I have not. On my DeLorean the OEM 
> TABs were both bent so badly that I had a really tough time getting them 
> out. I replaced them with a much stronger bolts (it was not a Toby TAB, but 
> a similar bolt) and guess what.... both bolts were bent again after less 
> than 10.000 miles. That is a bad and dangerous design! 
> 
> BTW, do those TABs hold up ok in case of the Esprit? 
> 
> You will see what I have come up with as soon as it materializes. For now 
> it's just a pile of metal parts that will become the frame and suspension ;) 
> 
> Take care, 
> 
> Tom Niemczewski 
> vin 6149 (in Poland!) 
> Google earth: 52°25'17.30"N 21° 1'58.00"E 
> dmctom@xxxxxxxxx 
> www.deloreana.com 

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